Sorry, forgot about those things. I would need a PSU, I don't think I'd need a the optical drive- since I can install XBMC with a flash drive. Yes to the case, something that's not too large- so it can fit comfortably near the receiver.
Also, a generic question about XBMC. If the .mkv has 720 video and 5.1 sound in it, will the HTPC be able to send that to the receiver properly, or will I be running into some sort of issue.